This invention relates to bandlimited telephone line data communication and in particular to digital data transmission and reception employing phase, amplitude and frequency modulation techniques in connection with frequency domain multiplexing.
Digital data is generally communicated at bit rates in the range from 300 bits/sec. to about 9600 bits/sec. over the public voice telephone switched network in either full duplex or half duplex operation. The effective bandwidth of a standard voice grade telephone line is from about 300 Hz to 3500 Hz with most energy concentrated between about 500 Hz and 2800 Hz. Typical modulator-demodulators (modems) operating on telephone lines, which are designed to provide 1200 bits/sec. full duplex communication across a single two-wire telephone line, divide the available bandwidth in half. The lower portion of the band (from about 300 Hz to 1900 Hz) may be used by the originating modem to transmit a single channel, while the higher frequency portion of the band (between about 1900 Hz and 3500 Hz) may be used for transmission of a single channel by the answering modem. Modems that employ this technique generally have a single carrier signal centered in each band portion which is modulated with frequency deviations within the available bandwidth.
Various modulation techniques are known for encoding digital information on a basically analog carrier system. One of the more popular is called quadrature amplitude modulation, which encodes information in the form of a combination of amplitude and phase relative to a reference signal. Other modulation techniques are known, such as differential phase shift keying (DPSK), frequency shift keying (FSK) and polybinary amplitude modulation.
In order to increase the rate at which data is transmitted, some modems use several different carrier frequencies within the available bandwidth, with each carrier frequency separately modulated. The several carrier frequencies are combined and transmitted simultaneously in a group. Such groups (called baud intervals) are transmitted serially to produce a data stream. One such modulator is shown in Stenstrom et al., U.S. Pat. No. 4,100,369. A demodulator for such a modulation method is shown in Daguet et al., U.S. Pat. No. 3,891,803. These systems have the disadvantage of being susceptible to intersymbol interference between such groups due to the abrupt change at the edge of the baud intervals and also due to the large changes in the nature of the signals within each baud interval. In addition, group delays due to transmission line delays affect different frequencies by different amounts, with the largest effects at baud edges.
The known method for reducing such intersymbol interference is to provide spacing between baud intervals. The demodulator ignores the signals in the space between baud intervals and thus transients in such spaces have no effect. This method is shown in Hick, Digital Super Modem: Why and How It Was Developed, DATA COMMUNICATIONS 87 (June 1980) and in Keasler et al., U.S. Pat. No. 4,206,320. This method, however, is not effective for full duplex operation over a two wire line because the transients in the transition spacing between baud intervals in one direction will interfere with the baud intervals in the other direction. In addition, the use of transition spacing limits the amount of data that can be transmitted on the available bandwidth.